Obesity is recognized as a major public health concern and has become a national health priority. With the prevalence of adolescent obesity nearly triple what it was two decades ago, effective interventions to prevent and treat overweight and obesity among adolescents are needed. Programs to enhance the healthy behaviors related to obesity prevention with cost-effective, science-based and easily deliverable interventions can be effective solutions to help curb the rate of childhood overweight and obesity. The objective of this research is to develop and test a Transtheoretical Model-based intervention for high school students that promotes the adoption and maintenance of healthy behaviors related to the prevention of obesity. The successful completion of our Phase I project demonstrates that a stage-matched, multi-media expert system is both acceptable to high school students and feasible to conduct in a high school setting. The primary goals of our Phase II application are to complete the development of the expert system intervention and to assess its effectiveness in a randomized clinical trial. Effectiveness will be assessed by comparing fruit and vegetable consumption, physical activity, TV watching, and several secondary outcome measures among high school students randomly assigned to the intervention or control condition.